Thermal image converter system



April 17, 1962 R. c. OHLMANN ET AL 3,030,546

THERMAL IMAGE CONVERTER SYSTEM Filed Deo. 23, 1957 United States Patent ffice 3,030,546 THERMAL IMAGE CONVERTER SYSTEM Robert C. hlmann, Albany, Calif., and Max Garbuny, Pittsburgh, Pa., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Air Force y, Filed Dec. 23, 1957, Ser. No. 704,843

2 Claims. (Cl. 315-9) This invention refers to the conversion of thermal images to visible screen images and more particularly to such conversion with automatic compensation for variations in the sensitivity of the photothermionic surfaces or in other factors, such as the emission coeflicient of the scanning phosphors.

In image conversion systems of the type using a thermal image on a photoemitter and a light spot for scanning the photoemitter surface, serious problems are encountered in achieving faithful image reproduction. Some of these problems are due to physical nonuniformities in the sensitivity of the photoemissive surface or the phosphors producing the scanning spot or both. For example, the scanning spot of light is generally obtained from a phosphoresceut screen of a cathode ray tube and projected by a lens system onto the photoemitter surfaceearrying the thermal image. A variation of intensity of the scanning light spot will result in a corresponding variation in electron emission from the photoemitter. Since the thermal image should be the sole electron emission controlling factor, such Variations in scanning light spot intensity produce undesired, erroneous, image reproducing electron emission response. Similarly, variations in sensitivity of the photoemitter surface due to physical nonunlformities cause corresponding erroneous electron emission responses to the scanning light spot.

Heretofore it has been attempted to confine image reproducing, electron emission response to the thermally induced variety by limiting the scanning phosphors to those having the characteristic of emitting light rays of relatively long wavelengths. The scanning light was conned to long wavelengths in order to limit the liberation of electrons of the photoemitter to those in the thermally sensitive energy levels. Thus for a cesium antimony photoemitter surface, red phosphors have been used to obtain a long Wavelength scanning spot. However, the resulting long wavelength scanning light has an undesirable slow response.

Pursuant to the present invention the `above problems have been overcome by adoption of image converting methods and means wherein the photothermionic surface is scanned by a flying spot created by light rays of relatively short wave-length, and wherein the resulting electron emission from the photothermionic surface is utilized in two dilerent ways, namely: (a) that portion of the emission which is thermally induced is carried to the receiving kinescope of the system by Way of the collecting electrode of the photothermionic device and an amplilier circuit, while (b) that portion of the emission which is thermally insensitive is periodically--that is, during selected time cyclesprevented from reaching the collector electrode by electrical restraining means in the form of a biasing voltage `applied to such emission; on the other hand, this non-thermal emission is, in alternate time cycles, allowed to ow to the collector electrode by the electrical operation of positively pulsing the biased grid to remove its flow-inhibiting bias and thus permit the nonthermally induced electron flow. This pulse-induced nonthermal emission, being thus added to the normal thermally induced emission, combines therewith in an amplitude comparison circuit to produce a correction voltage that is thereupon applied to the control grid of the iiying spot-creating cathode ray kinescope, where it is elective 3,030,546 Patented Apr, 17, 1962 vto modulate the ying spot characteristics and thereby control the photothermionic output These and other objects, features and advantages of the invention will become more apparent from the following description taken in connection with the accompanying drawing illustrating a preferred embodiment of the invention wherein an infrared image 9 is focussed onto the photoelectric screen 31 of photothermionic tube 10 by reflector 16 forming a temperature image corresponding to the temperature pattern of the observed scene. Auxiliary kinescope 12 produces a light beam which is focussed onto the photoelectric screen of tube 10 by directive lens system 15. The light beam or scanning spot is controlled by deiiection coil 14, electrically activated by scanning circuit 28 which also activates dellection coil 13 of the receiving or viewing kinescope 11.

The master pulse generator 22 provides pulses` through bias supply 21 to grid 19 of photothermionic tube 10, through line 30 and bias switch and level control 27 to grid 18 of auxiliary kinescope 12, and through switch circuit 24 to grid 17 of receiving kinescope 11.

Electrons are collected on collector electrode 20 of photoelectric tube 10, fed to Wide Band Amplifier 23, then through switch circuit 24, to amplitude comparison circuit 25 and compared to standard level 26. The difference between the fixed standard and the output of tube 10 in ramplitude and polarity is fed to the grid 13 of scanning tube 12 through the bias switch and level control 27.

The output of photoelectric tube 10 is amplified in Wide Band Amplifier 23 and modulates viewing kinescope 11 through switch circuit 24 and grid 17.

Sensitive surface 31 is scanned with light of relatively short wavelength which causes photoemission of both thermally sensitive and insensitive electrons. A grid 19 between the sensitive -surface 31 `and the collector 20 has a negative bias upon it suiiicient to prevent all but thermally sensitive electrons from reaching the collector. However, a positive pulse is applied to the grid at regular intervals to remove the bias and allow thermally insensitive electrons to flow to the collector. The resulting image signal varies with the incremental sensitivity of the infrared sensitive surface, and is used to derive a control potential to compensate for such variations. Pulsing of the retarding field allows discrimination between photoelectrons produced by infrared and the photoelectrons due to the scan of the light beam. This allows correction of the infrared induced photoelectrons for fluctuations in the total emission caused by nonuniformities of either the phosphor light source or the photoemissive surface. The magnitude of the current due to the thermally insensitive electrons will depend upon the sensitivity of the surface at the position of the scanning spot at the time of the pulse. This current pulse will then be made to control the thermally sensitive electron current during the next part of the cycle when the bias is present. This control can be accomplished by any of a number of methods, some of which are described below. A switch circuit 24 in series with the intensity control 17 of the receiving kinescope 11 allows the intensity to be modulated only by the thermally sensitive electron current which is then independent of the non-uniformities of the sensitive surface or of the scanning phosphore.

While there is novelty in the combination and interrelationship of the components illustrated in the drawing, and in the mode of operation of the system incorporated therein, and their functional effect, each upon the others, many of said components are not novel, per se. Thus, for example, pulse generator 22 may be of conventional construction comparable to that of the pulse generator 72 illustrated in the drawings of U.S. Patent No. 2,611,820 issued to Frank I. Somerson, Sept. 23, 1952; similarly,

3 switch circuit 24 may be of conventional construction, comparable to that of any conventional on-ofT electronic switch that may be operative normally to pass electric current from element 23 to element 25, but will be cut off, 'periodically and momentarily, each time generator 22 transmits a pulse thereto. By the same token, amplitude comparison circuit 25 may be any well-known type of difference amplier-see, for example, the difference amplifier illustrated in U.S. Patent No. 2,780,682 issued to Gerrit Klein on Feb. 5, 1957; and standard voltage level 26 may be any conventional battery or current generator capable of maintaining its current output at constant voltage, for Vstandardization purposes. Likewise, switch 27 may be of any conventional coincidence tube construction, with dual input grids, as described, for example, by Terman in Electronic and Radio Engineering (4th edition), at page 659, said text having been published by -McGraW-Hill in 1955. Amplier 23 may be any `D.C.

amplifier, of which there are countless well-known varieties.

With the application of a pulse to the grid 19, as above described, there is a simultaneous pulse transmission to the intensity control 18of the auxiliary kinescope 12 to reduce the intensity of the light spot. This is necessary since the thermally insensitive electron current would be much greater than the thermally sensitive current if this were not done, which might make control of each diicult in the lsame circuitry. The magnitude of this current controls the intensity of scanning spot during the next period of thermally sensitive current ow. Thus if the surface becomes less sensitive in a particular region, the thermally insensitive current pulse will be smaller, which would decrease the control grid voltage, and increase the beam current in the auxiliary kinescope 12 and increase the brightness of the scanning spot suciently to compensate 4 for the decrease sensitivity of the photoemitter or the scanning phosphors.

What is claimed is: 1. In combination, an image tube having an element sensitive to photo and thermal stimulus, said tube also having 'a control grid and an electron collector electrode;

`means for applying a thermal image to said sensitive element; light means for scanning said element to produce photo-response current ow to said electrode; means coupled to said control grid for producing a biasing voltage discriminating against the non-thermally stimulated portion of said photo-response current ow; means coupled to the biasing means -for periodically removing said bias voltage; and means in response relation to the last-mem tioned Vmeans for controlling the intensity of said light means in accordance with the density of electron flow to said electron collector electrode.

2. A system as defined in claim 1, including kinescopic means for visually displaying an illuminated image representative of said thermal image, and wherein said light means is constituted by auxiliary kinescopic means having a control grid energized by a voltage representative of the difference lbetween a preselected voltage level and a voltage derived from the output of said image tube, as taken 01T said collector electrode.

References Cife'dvin the are of this parent UNITED sTATEs PATENTS 2,188,679 Dov'a'ston et al. Jan. 30, v1940 Vall Mierlo Sept. 16, 2,611,820 Somers a f Sept. 23, 1952 2,804,498 Theile Aug. 27, 1957 2,938,141 Garbuny et al. --.7 May A24, 1960 2,979,622 Garbuny a Apr. 11, 1961 

